Folds Faults and Mountains

Fold and Thrust Mountains Enormous mountain ranges form when plates converge. Contorted rocks show the power of plate tectonics.

Rock Distortion

Convergent Plate Boundaries and Folding Continent-Continent collision forms Folded Mountain Belt:Alps, Himalayans, Appalachians Ocean-Ocean collision forms Island Arc: Japan, Aleutians, Cent. Am.

Evidence of Lateral Compression Formerly horizontal layers are twisted, bent, or broken. Some folded rocks are pushed over on their sides, or even upside down.

Source: Martin Bond/Science Photo Library/Photo Researchers, Inc. Folded Sandstone Source: Martin Bond/Science Photo Library/Photo Researchers, Inc.

Studying Faults and Folds The branch of geology that studies crustal deformation is called Structural Geology. Geologic structures determine ground stability, and where to build cities.

Stress Units are Pressure: Force/Area Three types of stress a) Compression causes bending b) Tension causes thinning Shearing causes one type of faults

Compression, Tension, and Shearing Stress Convergent Divergent Transform

Types of deformation Elastic deformation up to elastic limit Springs back to original shape Demo: Pencil Brittle failure (it breaks) Demo Pencil Causes: 1. subjected to great stress that exceeds the yield point AKA elastic limit, OR Subjected to sudden stress AKA “impact” Plastic deformation Does not spring back … keeps deformed shape Demo Chewing gum Cause can be high temperature – near melting or high pressure … squeezed like a ball of clay

Relationship Between Stress and Strain Strain can be a change in shape (a deformation) due to an applied stress

Relationship Between Stress and Strain at low Temps and Pressure or Sudden Stress

Relationship Between Stress and Strain under high Temps or Pressure

Factors affecting rock deformation Intensity of applied stress Heat –Temperature of the Rock Amount of Time the Stress is applied Rock Composition

Interpreting Deformed Rocks Most apparent in sedimentary rocks Importance of deformation Indicates past plate motions Indicates other past geological events Locates specific natural resources Mapping 101: Rock orientation: strike and dip

Strike and Dip Strike is long line, dip is short line Note the angle of dip given 45o Strike intersection w horizontal, dip perpendicular, angle from horizontal down toward surface

Folds Folds def: Bends in rock layers Types: synclines and anticlines Syncline (downfold) innermost rocks youngest Anticline (upfold) innermost rocks oldest Parts of a fold (limbs, axial plane, axis) Note: Anticlines and synclines are structures in rocks, not surface landforms

Folded Rocks, Hwy 23 Newfoundland, New Jersey Note highest point Adjacent Anticline and Syncline Source: Breck P. Kent

Folded Rocks (Dorset, England) Center has overturned area Older Overturned Area Younger Lucky we have ways of recognizing right side up What are they? Source: Tom Bean Younger Older

Folded Rock Before Erosion

Folded Rock After Erosion Eroded Anticline, older rocks in center. Syncline is opposite.

Topography may be opposite of Structure Anticline Before/After Erosion Notice center rock oldest

Topography may be opposite of Structure Syncline Before/After Erosion Notice center rock youngest

Fold symmetry a) Symmetrical or open folds b) Asymmetrical folds c) Overturned folds d) Recumbent folds e) Plunging folds

Various Folds

Various Folds (cont'd)

Various Folds (cont'd)

Various Folds (cont'd) Not a good drawing, axial plane should be closer to horizontal

Plunging Folds Up End Down End Demo: Plastic box, water, paper folds Up End Down End Nose of anticline points direction of plunge, syncline nose in opposite direction

Source: GEOPIC©, Earth Satellite Corporation Plunging Folds Source: GEOPIC©, Earth Satellite Corporation

Interpreting Folds Determine if center rocks are older or younger than flanks: fossils, right side up clues (graded bedding and mudcracks) Are limbs parallel or “Nosed”? Determine limb dips from measurements, stream V’s. Strike and Dip Use nose rules for anticlines and synclines

Again: Strike and Dip

Domes and basins 1. Domes 2. Basins 3. Occur within plates 4. Result from vertical forces 5. Geographic examples (later)

3-D: Dome and Basin

Fractures Fractures - Joints: fractures with no relative movement - Faults: fractures with relative movement

Joints: Fractures – with no movement Source: Martin G. Miller/Visuals Unlimited

Fault Type 1 - Dip-slip faults 1) Terms: Hanging wall and footwall 2) Normal faults (a) Grabens (b) Horsts 3) Reverse faults a) low angle called Thrust faults 4) Oblique-slip faults

Dip-Slip Faults

Normal Fault: Hanging Wall Down Hanging wall overhangs the fault plane Key Bed Source: John S. Shelton Especially common in divergent margins

Normal Fault (Hanging Wall down)

Reverse Fault (called “Thrust Fault” if shallow angle) Typical of convergent margins (Hanging wall Up) Structural Geology is taught by Dr. Krall Younger What phase of magma fractionation would result in the placement of this ore body? Which formed first, the ore body or the fault? What common mineral is mostly likely in the ore body? Miners pay geologists to find their lost orebody One friend earned enough to buy a house This poor guy is out of luck

Evidence of faults a) Visible displacement of rocks b) Pulverized rock and “Slickensides” c) Key beds cut out by faulting reappear elsewhere.

Fracture Zones and Slickensides http://pangea.stanford.edu/~laurent/english/research/Slickensides.gif

Types of Faults - 2 Strike-slip faults 1 1) Example: San Andreas Transform fault Distinctive landforms (linear valleys, chains of lakes, sag ponds, topographic saddles) Fresh pulverized rock. Transform fault through granite: Arkose sandstone Evidence of Shear stress

Source: Georg Gerster/Wingstock/Comstock San Andreas Fault Source: Georg Gerster/Wingstock/Comstock

Horizontal Movement Along Strike-Slip Fault

Oblique Slip Also seen in Transform Faults such as San Andreas Both strike slip and dip-slip Also seen in Transform Faults such as San Andreas

Types of faults Strike-slip faults 2 1) Example: Mid-Ocean Ridge Transform faults Small offsets in ridge San Andreas is also ridge offset, but on a huge scale with a historical twist

Faults & Plate Tectonics Divergence Convergence Transform

Plate tectonics and faulting Normal faults: mid-ocean ridges and continental rifts are the same thing. Divergent Margins Surface rock is pulled apart Hanging wall drops down

Horst and Graben Formation

Horst and Graben Formation

Source: Simon Fraser/Science Photo Library/Photo Researchers, Inc. Graben in Iceland Source: Simon Fraser/Science Photo Library/Photo Researchers, Inc.

Plate tectonics and faulting Reverse and thrust faults: convergent plate boundaries Hanging Wall is pushed up.

Lewis Thrust Fault

Lewis Thrust Fault (cont'd)

Lewis Thrust Fault (cont'd) Source: Breck P. Kent PreCambrian Limestone over Cretaceous Shales

Plate tectonics and faulting c) Strike-slip faults: Transform Boundaries

San Andreas Fault

Types and processes of mountain-building (Orogenesis) 1. Volcanic mountains 2. Fold-and-thrust mountains 3. Fault-block mountains 4. Upwarped mountains

Types of Mountains 2. Fold-and-thrust mountains Formed by Continent-Continent Collisions

Appalachian Mountain System

Model for the Evolution of the Southern Appalachians Supercontinent breaks up, rifts apart. rift Another Rift Over here somewhere Another rift starts moving Africa west. The ocean floor breaks and one side subducts, starting a new island arc.

Model for the Evolution of the Southern Appalachians (cont’d) The ocean floor breaks again, new subduction adds volcanics to an existing microcontinent Weak rifts Net westward movement pushes the ridge, subduction zone and fragment into N.America Rifting restarts to the East

Model for the Evolution of the Southern Appalachians (cont’d) Arc and subduction zone collide w/ N.Am., westward subduction starts The continents collide

Model for the Evolution of the Southern Appalachians (cont’d) Rifting Restarts

Collisional Mountains ??? (The Grand Tetons in Wyoming) Source: Peter French/DRK Photo Paradigm shifts: What is wrong with our model? More on this later

Fault-block mountains Rift Valleys, Mid Ocean Ridges Basin and Range province ??? Normal Fault Blocks as in East Africa Divergent Margins? Paradigm Shifts

Origin of the Basin and Range Southwestern North America Looks different Paradigm Shifts

Upwarped mountains a) Gently bent without much deformation b) Ascent of buoyant mantle material c) Far from plate boundaries d) Adirondack Mountains: Uplift of deep PreCambrian Igneous and Metamorphic rocks

The Adirondack Mountains of Northern New York Source: Clyde H. Smith/Allstock/Tony Stone Images

Anticlines and Oil Early USA petroleum exploration, e.g. Pennsylvania anticlines

Faults and Oil

End of Chapter 9